Rate distortion optimized document coding using resolution enhanced rendering

Raster document coders are typically based on the use of a binary mask layer that efficiently encodes the text and graphic content. While these methods can yield much higher compression ratios than natural image compression methods, the binary representation tends to distort fine document details, such as thin lines, and text edges. In this paper, we describe a method for encoding and decoding the binary mask layer that substantially improves the decoded document quality at a fixed bit rate. This method, which we call resolution enhanced rendering (RER), works by adaptively dithering the encoded binary mask, and then applying a nonlinear predictor to decode a gray level mask at the same or higher resolution. We present experimental results illustrating that the RER method can substantially improve document quality at high compression ratios.